The invention relates to a method for bonding or potting substrates by means of cationically polymerizable masses being activated before joining or potting by irradiation with visible light and curing after joining or potting of the substrates without the irradiation being continued.
Such methods have been known from EP-A-0 279 199 as well as from JP-A-61-98740, for instance. However, ultraviolet radiation is exclusively used therein for photo-activating the cationically polymerizable masses. At an UV-irradiation period of 10 sec, the known masses gelate after 20 to 30 s at room temperature, but after less than 10 s at 100.degree. C.
Since UV-curable masses harden quickly on the locations accessible to UV-radiation, but do not harden at all or much more slowly in the regions being less accessible or masked, a second hardening mechanism is necessary which is caused by initiators of the thermal polymerization and requires higher temperatures. However, the temperature load being related to thermal polymerization reduces the statistical life expectancy of the substrates in many fields of application, for instance in bonding or potting electronic components.
The above-mentioned references had proposed to irradiate the UV-curable masses in the application means, that is, before they are applied onto the substrate and to supply the masses already being photo-activated to the substrate or a casting mould only then for reducing the problems related to inhomogeneous photo-activation due to UV-irradiation. This results in uniform radiation over the whole mass cross-section whereas it is avoided that masked regions appear. However, the drawback inherent to these methods consists in that the pot life of the photo-activated masses is shortened too much for many fields of application. Since the masses already gelate 20 to 30 sec after UV-irradiation is finished, the pot life is considerably shorter than 20 to 30 sec as a marked reduction of initial adhesiveness, a marked reduction of wetting ability and a marked increase in viscosity can be noticed in the masses even before the onset of gelification.
Moreover, the UV-curable masses used in the known methods are sensitive with respect to atmospheric oxygen. Further, strongly dyeing the adhesive mass as is often demanded for chip adhesives may reduce the depth of penetration of the UV-radiation, consequently prevent complete hardening and later result in corrosion phenomena.
It has been known from DE-PS 26 18 897 as well as from DE-OS 30 35 807 and 30 46 034 to use cationically polymerizable masses for bonding or potting substrates which are hardened under the influence of visible light situated in the wavelength range of 380 to 800 nm. These masses contain as photo-initiators diaryl iodonium salts or triaryl sulphonium salts respectively together with a photo-sensitizer in the form of a cationic or basic dye. Irradiation times of 5 to 10 min are specified in the examples of DE-OS 30 46 034. Complete hardening of the masses irradiated in this manner occurs at room temperature 1 to 3 hours after irradiation has finished.
From DE-OS 36 43 400 and 37 24 838, there have also been known methods for bonding substrates by means of cationically polymerizable masses being activated before joining by irradiation with light in the wavelength range of 200 to 700 nm and curing without further irradiation when being left to stand in the dark. These masses had been irradiated for about 1 to 3 minutes and left to stand until complete hardening under a pressure of 1 to 200 MPa at room temperature for 2 to 36 hours. The gelification point of the masses had been reached after 210 sec., calculated from the beginning of irradiation. As photo-initiators, there had been used, in these masses, aryl sulphinates, diaryl iodonium salts and triaryl sulphonium salts, respectively together with sensitizers in the form of dyes or camphor quinone.